Microelectramechnical system (MEMS) sensors such as acceleration sensors, vibration gyroscopes and vibration sensors are conventionally known which MEMS sensors convert a displacement of a flexible beam coupling a mass body into an electrical signal. Silicon processing technologies have been developed highly along with the advancement of integrated circuits, and are suitable for manufacturing MEMS. Charge carriers (electrons, holes) in silicon are influenced by stress application. Electron mobility increases as a tensile stress along a transport direction increases, and reduces as a compressive stress along a transport direction increases. Positive hole mobility increases as a compressive stress along a transport direction increases, and reduces as a tensile stress along a transport direction increases. A surface of a semiconductor layer receives a tensile stress as the semiconductor layer is deformed in a convex shape, and receives a compressive stress as the semiconductor layer is deformed in a concave shape. Change in the mobility of charge carriers in semiconductor can be detected by fabricating a semiconductor device such as a resistor or a MOS transistor in a silicon region receiving a strong stress. For example, an acceleration is detected by detecting change in a resistance value (piezo resistor) caused by a stress. A mass body is coupled to one end of a flexible beam supported to a support or the like at the other end. Since the mass body has an inertia, as the support moves, the flexible beam is deformed and receives a stress. As a cross sectional area of the flexible beam is made small, a stress per unit cross-sectional area becomes large and deformation amount becomes large.
Generally, piezo resistors are formed on a flexible beam in order to detect acceleration. Considering the convenience of fabrication processes, it is desired that piezo resistors and wirings are formed on a front surface of a flexible beam. In order to improve detection precision, a bridge circuit is generally formed by four piezo resistors. For example, four piezo resistors are formed at both edges in a width direction, near at opposite ends of the flexible beam. A resistor increasing a resistance value and a resistor reducing a resistance value upon application of a certain stress are serially connected. Two serial connections are connected in antiparallel direction between power supply wirings. Voltage difference between interconnection points in the respective serial connections is detected.
There is an acceleration sensor for detecting acceleration in one-dimensional direction as well as acceleration sensors for detecting acceleration in two- or three-dimensional directions. It is desired that a one-dimensional acceleration sensor detects acceleration only in a predetermined direction, and does not respond to accelerations in other directions.
JP-A-8-160066 proposes a cantilever type acceleration sensor constituted of a fixed portion, a weight movable by acceleration, a flexible beam connecting the fixed portion and the weight, and resistor elements disposed in the flexible beam, formed by processing a silicon substrate, wherein the flexible beam is narrower than the weight in a width direction perpendicular to a thickness direction of the silicon substrate, and has the same thickness as the weight in the thickness direction of the silicon substrate. A center of gravity of the weight is aligned on an extension line of the center line of the flexible beam. The flexible beam is deformable in the width direction in in-plane directions of the substrate surface, and deformation by acceleration in the longitudinal direction of the flexible beam is suppressed. Two pairs of resistor elements are formed in a flexible beam surface area at both edge regions adjacent to the fixed portion and to the weight. As the flexible beam is inflected and deformed in the width direction, two resistor elements at one edge are compressed, and two resistor elements at the other edge are stretched.